Voltage references are used widely in electronics to create a stable current or voltage. The voltage references are normally chosen either to be fixed, or to track temperature or process variations.
Most of the known circuits which generate a reference voltage use a structure of the bandgap type which is based on the principle of the thermal dependence of both the voltage drop across the base-emitter junction, VBE, and the thermal voltage VT (VT=k*T/q) of a bipolar transistor. These two voltage values have a negative and a positive thermal coefficient, respectively, i.e. they are respectively increased and decreased with the device temperature. Thus, by an appropriate weighted sum, it is possible to obtain from them a voltage generating circuit which has a null thermal coefficient, i.e. is unaffected by variations in temperature. The circuit of this type is disclosed in U.S. Pat. No. 5,747,978 for generating a reference voltage and detecting a drop in a supply voltage.
Another widely used approach is to implement a replica bias structure. According to U.S. Pat. No. 5,847,616, a replica biasing circuit producing the impedance control voltage immune to variations in the semiconductor manufacturing process, fluctuations in the power supply voltage source, and changes in operating temperature is provided for a voltage reference generator to produce a stable reference voltage that is coupled to the voltage reference input terminal of the replica biasing circuit.
The above approach is typically used also in differential current mode inverters/buffers such as shown in FIG. 2, to provide a predefined current through the MOSFET 1 and another one for the pull-ups. It can be used also to control of pull ups in single ended open drain logic and in totem pole logic, etc.
Whilst the present invention has wide application, for the sake of clarity, consider the case where a voltage reference is required in a differential current mode inverter/buffer as shown in FIG. 2.
This circuit in FIG. 2 has a voltage reference 22 to control the current in the MOSFET 21, and a further voltage reference 20 is required to control the voltage swing of a MOSFET for the pull up devices 25 and 26.
These voltage references are normally chosen either to be fixed, or to track temperature or process variations. Usually the voltage reference has the same structure as the transistor to be controlled, with constant logic levels, controlling the logic low voltage level based on the required swing. However, in this case, all variations in temperature, process technology and power supply cause variations in the propagation time of the gate.
Libraries of leaf cells used in the design of integrated circuits quote a range of propagation delays for each cell. At the same time, it is not usually of any advantage when the cell operates faster than the slowest case: circuits must be simulated using worst case figures.